This invention relates to a data display control system and more particularly, to an improvement of a data display control system for use with a display device such as a CRT (cathode ray tube) display for displaying data thereon which is operable in various display modes including a selective picture display, a superimposed picture display and a shifting picture display.
In the past, it has already been proposed to superimpose a data information to be displayed on a CRT screen 11 upon a data information to be displayed independently of the former information on a CRT screen 12 in order to display a superimposed picture on a CRT screen 13, as shown in FIGS. 1a to 1c. For such a superimposition, a CRT display device is usually operated with an arrangement as shown in FIG. 2. More particularly, a data information (corresponding to that shown in FIG. 1a) and a second data information (corresponding to FIG. 1b) to be superimposed upon each other are respectively memorized into two refresh memories 24 and 25 from a CPU (central processor unit) via an interface 21 under the control of a read/write (R/W) control circuit 22 and a timing control circuit 23, and these data synchronously read out of the two memories 24 and 25 are composed in a video signal forming circuit 26 and transmitted to a viewer 27 comprised of a CRT display which, in turn, displays thereon a superimposed picture as shown in FIG. 1C. In FIG. 2, symbols AO to A1919 designate address numbers.
Alternatively, on the other hand, an approach has been made to a vertical or horizontal shift of the contents displayed on the screen, which approach is typically practicised as follows. As shown in FIG. 3, there is provided a refresh memory 35 for receiving and memorizing data commensurate with three picture frames from a CPU via an interface 31 under the control of a read/write (R/W) control circuit 32, the refresh memory 35 having continuous adresses AO to A1919, A1920 to A3839 and A3840 to A5759 for the three picture frames. In order that pictures displayed on a viewer 37 are shifted on the basis of a shift instruction from the CPU or a keyboard, a start address designating circuit 34 for designating a start address of the refresh memory on the basis of the shift instruction is incorporated in a timing control circuit 33. When a shift instruction occurs, reading of data information from the memory 35 is started at an address designated by the start address designating circuit 34 and the data information thus read out, following conversion into a video signal in a video signal forming circuit 36, is transmitted to a viewer 37. The data read out are successively displayed on the screen of viewer 37 in such a manner that the data information corresponding to the start address is positioned at the beginning of the screen. Thus, by changing the start address, the vertical or horizontal position of the data information displayed on the screen can be changed so that a so-called picture shift can be accomplished.
Incidentally, the aforementioned two prior art systems solely perform one of the two display modes, superimposition and shift of picture, and have no capability of practicising both the display modes selectively and independently of each other. Also, a recent trend demands availability of a CRT display device the display modes of which can be selected optionally by users.